The invention relates to a method of transmitting data in a radio system from a transmitter to a receiver, and to a radio system, a radio transmitter and a radio receiver using the method. The use of the method is described in EGPRS (Enhanced General Packet Radio Service).
EGPRS (Enhanced General Packet Radio Service) is a system based on GSM (Global System for Mobile Communications) utilising packet- switched transmission. EGPRS employs EDGE (Enhanced Data Rates for GSM Evolution) technique in order to increase data transmission capacity. In addition to GMSK (Gaussian Minimum-Shift Keying) modulation normally used in the GSM, 8-PSK (8-Phase Shift Keying) modulation can be used for packet data channels. The purpose is mainly to implement non-realtime data transmission services, such as copying files and use of an Internet browser, but also real-time services as packet-switched services for transmitting speech and video, for example. In principle, data transmission capacity can vary from a few kbit/s up to 400 kbit/s.
Also other procedures are used in order to increase capacity, for example blind detection of modulation, link adaptation and incremental redundancy.
Blind detection of modulation means that it is not necessary to signal to a receiver which modulation method is being used but the receiver detects the modulation method when it receives a signal.
Link adaptation refers to changing the code rates of blocks to be transmitted on the basis of measurements carried out on the channel. The code rate can be changed between retransmissions of the same block. Another alternative is to change the code rate between successive blocks, provided, however, that all transmissions of a single block are coded by the same code rate. The purpose is to optimize the use of radio resources taking instantaneous variations in the conditions of the radio interface into account. The purpose is to optimize user data throughput and to minimize delay.
The code rate of a block refers to the ratio of the number of user data bits to the coded data bits of a channel. If, for example, 100 user data bits are coded into 200 data bits to be transmitted over the channel, the code rate obtained is 100/200=xc2xd.
FIG. 3A shows examples of changing the code rate of a block. In FIG. 3A, a data block to be transmitted is shown above the X-axis, and blocks transmitted actually over the radio link are shown below the X-axis. The Y-axis denotes passage of time. Block sizes are scaled in accordance with each other, i.e. the larger the block, the more bits to be transmitted said block comprises.
A block A 300 is to be transmitted over the radio link. First transmission 302 fails, so the transmission is repeated 304. Link adaptation was not carried out, since as can be seen from FIG. 3A, the blocks 302, 304 are equal in size. The code rate in both transmissions 302, 304 is 1.
In comparison with the transmission of the block A 300, link adaptation is carried out in the transmission of a block B 306 by changing the amount of user data. Compared with the block A 300, it is detected that the size of the block B 306 is reduced by half. The code rate of a block 310 to be transmitted has been reduced to xc2xd.
Another way to carry out link adaptation is to change the size of a data block to be transmitted over the radio link. Compared with the transmission of the block A 300, in the transmission of a block C 312 link adaptation is carried out by changing the size of the data block to be transmitted. The code rate of a block 316 to be transmitted is xc2xdsince the size of the data block to be transmitted over the link has been doubled.
Under favourable conditions, for example, coding can be reduced, which means that more user payload can be transferred. Similarly, one modulation method can be better suited than the other to certain conditions on the radio interface. Different combinations of modulation and channel coding can be called modulation and coding schemes MCS.
If the coding conditions of a channel change extremely rapidly, it is impossible for the system to optimally select the code rate for the forthcoming transmission in advance. Incremental redundancy enables better adaptation to changing conditions. In incremental redundancy, a receiver is equipped with a memory to store the bits of radio blocks that have been received erroneously. Retransmitted radio blocks are then combined with the stored radio blocks, whereafter the receiver attempts to decode the block. Since there are more coded channel data bits to be used for decoding after the combining and the number of user data bits remains the same, the effective code rate of the block is decreased after retransmission, which makes decoding more feasible. An example of such a protocol is the hybrid FEC/ARQ (Forward Error Correction/Automatic Repeat Request), which uses error correction coding in order to decrease the number of retransmissions.
The effective code rate of the channel is adapted automatically since the channel conditions determine the number of necessary retransmissions, which in turn determines the code rate. FIG. 3A shows the simplest retransmission method for a data block D 318 to be transmitted. An original transmission 320 is carried out by a code rate 1, and a first retransmission 322 also by a code rate 1. After the first retransmission the code rate of the combined data block is xc2xd. A second retransmission would yield a code rate ⅓, a third retransmission a code rate xc2xc, and this could be continued until it would be possible to decode the combined data block.
The problem with the retransmission method disclosed is that the effective code rates are quantized with relatively large steps: after one retransmission the code rate is only half of the original. This means that the capacity of the system is wasted since a smaller reduction in the code rate would often be sufficient. A solution that has been provided discloses a method wherein the data block to be transmitted is divided into sub-blocks, for example into two sub-blocks, the number of the sub-blocks being denoted by D, which is described in FIG. 3A by a block E 324. The code rate used in the transmission of an original block 326 is 2. After a first retransmission 328 the code rate is 1, after a second retransmission 330 the code rate is ⅔, after a third retransmission the code rate would be xc2xd, after a fourth retransmission the code rate would be ⅖. The drawback of this method is that even under ideal channel conditions transmission of at least D data block(s) is necessary before the data block can be decoded, i.e. the code rate must be 1 at most.
An object of the invention is thus to provide a method and an apparatus implementing the method so as to enable efficient simultaneous utilization of link adaptation and incremental redundancy. This is achieved by the method disclosed below of transmitting data in a radio system from a transmitter to a receiver, the method comprising: channel coding a data block into a coded data block by using a selected channel coding; puncturing the coded data block by using a first puncturing pattern; transmitting the coded data block punctured by the first puncturing pattern to the receiver; detecting a need for retransmission of the received coded data block; transmitting a retransmission request of the coded data block to the transmitter. The method further comprises: increasing the code rate of the coded data block to be retransmitted by puncturing the coded data block coded by the channel coding of the original transmission by using a second puncturing pattern comprising fewer symbols to be transmitted than the first puncturing pattern; transmitting the coded data block punctured by the second puncturing pattern to the receiver; combining the received coded data block punctured by the first puncturing pattern and the received coded data block punctured by the second puncturing pattern; and decoding the channel coding of the combined coded data block.
The invention also relates to a radio system comprising: a transmitter and a receiver having a radio connection to the transmitter; the transmitter comprising a channel coder for channel coding a data block into a coded data block by using a selected channel coding and for puncturing the coded data block by using a first puncturing pattern, and transmission means for transmitting the coded data block punctured by the first puncturing pattern to the receiver; the receiver comprising a channel decoder for decoding the received coded data block, means for detecting a need for retransmission of the received coded data block, and means for transmitting a retransmission request of the coded data block to the transmitter. The channel coder increases the code rate of the coded data block to be retransmitted by puncturing the coded data block coded by the channel coding of the original transmission by using a second puncturing pattern comprising fewer symbols to be transmitted than the first puncturing pattern; the transmission means transmit the coded data block punctured by the second puncturing pattern to the receiver; the receiver comprises means for combining a received coded data block punctured by the first puncturing pattern and a received coded data block punctured by the second puncturing pattern; the channel decoder decodes the channel coding of the combined coded data block.
The invention further relates to a radio transmitter comprising: a channel coder for channel coding a data block into a coded data block by using a selected channel coding and for puncturing the coded data block by using a first puncturing pattern; transmission means for transmitting the coded data block punctured by the first puncturing pattern to a receiver; means for receiving a retransmission request of the coded data block. The channel coder increases the code rate of the coded data block to be retransmitted by puncturing the coded data block coded by the channel coding of the original transmission by using a second puncturing pattern comprising fewer symbols to be transmitted than the first puncturing pattern; the transmission means transmit the coded data block punctured by the second puncturing pattern to the receiver.
The invention still further relates to a radio receiver comprising: reception means for receiving a coded data block channel-coded by a selected channel coding and punctured by a first puncturing pattern; a channel decoder for decoding the received coded data block; means for detecting a need for retransmission of the received coded data block; means for transmitting a retransmission request of the coded data block to a transmitter. The reception means receive the retransmitted coded data block whose code rate has been increased by puncturing the coded data block coded by the channel coding of the original transmission by using a second puncturing pattern comprising fewer symbols to be transmitted than the first puncturing pattern; means for combining the received coded data block punctured by the first puncturing pattern and the received coded data block punctured by the second puncturing pattern; a channel coder decodes the channel coding of the combined coded data block.
The preferred embodiments of the invention are disclosed in the dependent claims.
The invention is based on the idea that the same channel coding has been used in the coding of the originally transmitted data block and the retransmitted data block, and the code rates of the two transmissions are made to differ by using different puncturing. Hence, despite the different code rates, the data blocks can be combined.
The advantage achieved by the method and apparatus of the invention is that there is a sufficiently dense range of effective code rates to enable the code rate required by the channel conditions to be selected relatively accurately, which saves the valuable radio resource of the system.